The present invention relates to improvements in directivity and gain of an antenna element associated with a finite length reflector, which has wide-angle directivity.
Explaining now, by way of example, in connection with an antenna element associated with a finite length reflector in which a feed antenna consists of a 1/2 wavelength dipole antenna (i.e. a dipole antenna associated with a reflector), in the case where the area of the reflector is small as compared to a square of an operating wavelength (.lambda.), the effect of the reflector is not fully achieved, resulting in degradation of the directivity and lowering of the gain. Especially with respect to the H-plane (a plane of magnetic field) directivity, degradation of the directivity in the proximity of the directions of extension of the reflector, that is, in the proximity of .+-.90.degree. relative to the direction of the maximum direction is remarkable, and radiation power of -6 to -10 dB relative to the maximum value of radiation has been observed. Accordingly, in the event that this antenna element is used, for example, as an element in an array or as a primary radiator of a parabolic reflector antenna, large sidelobes would be generated in the proximity of the above-described directions, and it becomes a cause for degradation of a performance of the directional antenna.
An antenna element associated with a rimmed reflector which has been heretofore used to obviate the above-mentioned shortcoming is schematically illustrated in FIG. 1, in which are shown a reflector 101, a metallic rim 102 electrically connected to the reflector 101, a dipole antenna 103 serving as a feed antenna and a feeder 104.
In the illustrated case, in order to improve the directivity it is necessary to select appropriately the respective dimensions of a diameter (d) of the reflector 101, a length (l) of the metallic rim and a gap distance (s) between the reflector 101 and the dipole antenna 103. However, at present a design procedure for uniquely determining these dimensions is not clearly known, but they are empirically determined in practice, and so, the above-mentioned structure is inconvenient for use. Furthermore, there exists a problem with respect to increase of weight and manufacture resulting from the provision of the metallic rim 102.
Alternatively, an antenna element in which a dipole antenna 103 is disposed within a circular waveguide 110 as shown in FIG. 2, has been also known in the prior art. In this case, when a diameter (d.sub.1) of the circular waveguide 110 is one wavelength or less (i.e. when the antenna aperture is small), a high frequency current is made to flow along the inside wall surface of the circular waveguide 110 towards the antenna aperture by an electromagnetic wave excited by the dipole antenna 103, and because of the small antenna aperture, a current (I.sub.O) flowing from the inside wall surface of the circular waveguide 110 to its outside wall surface is generated at the antenna aperture. This current (I.sub.O) would flow inversely towards the reflector 101 and at the same time would radiate an electromagnetic wave, resulting in degradation of the directivity. Accordingly, an antenna element having the above-mentioned structure necessitates the additional provision of a countermeasure such as a Bazooka balun for preventing current from outflowing to an outside conductor of the circular waveguide 110, and hence the antenna element has the shortcoming that complexity in structure as well as increase of weight accompanying the provision of the circular waveguide, are brought about.